1. Field of the Invention
The present invention generally relates to an acoustic source useful in marine seismic exploration. More specifically, the present invention relates to a compact, marine acoustical source capable of producing a strong primary pulse while substantially suppressing or eliminating undesired secondary pulses derivative from said primary pulse.
2. Description of the Prior Art
In marine seismic exploration, it is often desirable to generate in a body of water overlying a geological formation, a series of strong acoustical pulses or waves. These pulses rapidly pass through the water and the formation and are reflected at the surface for eventual interpretation. Methods of marine seismic exploration using such acoustical pulses have long constituted a major tool for marine oil and gas exploration.
To achieve such pulses, certain seismic sources such as explosives, air guns, gas exploders, etc., are purposely fired deep under water. It is well known that such firing creates a gas bubble or cavity and that the water acquires oscillatory energy which generates acoustic pressure wavelets, each consisting of a desired "primary" acoustic pressure pulse having an amplitude which is especially useful for most seismic exploration work, and which is followed by an oscillating succession of undesired "secondary" (sometimes called "bubble") acoustic pulses of decreasing amplitude. In this context, the words "bubble" and "cavity" will be used interchangeably.
A variety of apparatus and techniques have been developed in the art to overcome the development of such secondary pulses. For example, one early mechanical technique attempted to prevent these secondary pulses from traveling vertically downward by substantially surrounding the source with a container or cage having perforations so that the expanding gas bubble would have to do work in order to force water through the perforations. According to such theory, the work done by the expanding gas bubble dissipated its internal energy so that the ensuing secondary pulse would have reduced amplitudes. This technique was employed in a seismic source trademarked FLEXOTIR. A serious limitation inherent in this technique was that the desirable primary pulses likewise became reduced in strength since they too could travel only through the available perforations. Also, the perforated cage used in such technique became subject to rapid deterioration due to the great stress to which it was subjected during operation.
Other prior art techniques have utilized the injection of air into the expanding bubble for the purpose of shaping the secondary pulses. An overview of such techniques and their associated limitations and disadvantages are described in U.S. Pat. No. 4,735,281 and U.S. Pat. No. 4,921,068 which are herein incorporated by reference.
It may generally be stated, however, that prior art apparatus designed to emit a satisfactory primary pulse have failed to incorporate the structures necessary to implement state of the art techniques directed toward the suppression of undesired secondary pulses. Further, prior art apparatus directed toward attempted solutions of the problem of bubble suppression have generally been inefficient, heavy, and cumbersome. This combination has made their use, on board retrieval and repair, difficult.
Additionally, the operation of such apparatus has generally been comparatively dangerous as caused by the need to pressurize the devices on board the seismic vessel before immersing. Such on board pressurization has been needed to prevent fouling by the entrance of seawater into the units prior to firing.